https://wccftech.com/diablo-4-art-book-pages-leaked/

It was only last week that the art book featuring work from the whole Diablo series, "The Art of Diablo", was leaked by German outlet Gamestar. While it was rumoured that Blizzard was set to announce Diablo 4 during Blizzcon last year, cancelling it due to development issues - eventually leading up to the infamous mobile phone incident, it seems that this year will be where Diablo 4 is announced.

Indeed, the fact that the art book, featuring artwork from Diablo 4, is ready to ship means that time is running short to officially announce the game. So yes, expect the news that Diablo 4 has officially been announced sometime over the coming day or two.